1. Field
The present disclosure relates to a nitride semiconductor light-emitting device and a method for producing the same.
2. Description of the Related Art
A nitride semiconductor made of GaN, InGaN, AlGaN, or the like is used as a material of a light-emitting diode (LED), a laser diode (LD), or the like. The nitride semiconductor is excellent in terms of heat resistance or environmental resistance, and therefore, is also applied to electronic devices.
Meanwhile, bulk crystal growth of the nitride semiconductor is not easy. Therefore, producing the nitride semiconductor substrate having practicable quality incurs an expense, and therefore, the nitride semiconductor substrate having practicable quality is very expensive. Therefore, as the growth substrate for a layer made of a nitride semiconductor single crystal (hereinafter, referred to as “the nitride semiconductor layer”), a single crystal sapphire substrate is widely put into practical use, and as a method for growing the nitride semiconductor layer, the following method is generally used. That is, a nitride semiconductor layer (for example, GaN layer) is epitaxially grown on the single crystal sapphire substrate by, for example, metalorganic vapor phase epitaxy (MOVPE).
In a case where the single crystal sapphire substrate is used as the growth substrate for the nitride semiconductor layer, a crystal defect, lattice strain, or the like due to a lattice constant difference between the nitride semiconductor and sapphire is generated. Therefore, in a case where the single crystal sapphire substrate is used as the growth substrate for the nitride semiconductor layer, the most general method for growing the nitride semiconductor layer is a method in which a low temperature growing buffer layer made of AlN, GaN, or the like is grown on the single crystal sapphire substrate at a low temperature of substantially 600° C., and then the desired nitride semiconductor layer is grown on the low temperature growing buffer layer at a high temperature of substantially 1000° C. or more (for example, refer to Japanese Unexamined Patent Application Publication No. 2000-277435). When the low temperature growing buffer layer is formed, the crystal defects are reduced, and the lattice strains are relaxed.
In the nitride semiconductor, atoms are strongly bonded to each other. Therefore, in order to improve the crystal quality of the nitride semiconductor layer, it is generally said that the nitride semiconductor layer is desirably grown at a high temperature of 1000° C. or more. An epitaxial growth of the nitride semiconductor layer is enabled by forming the low temperature growing buffer layer and growing the nitride semiconductor layer at a high temperature of 1000° C. or more and, as a result, an LED which is capable of emitting light from ultraviolet to green light, is widely put into practical use.
Light-emitting devices including the nitride semiconductor layer (nitride semiconductor light-emitting device), particularly, typical lighting LEDs or blue LEDs for backlighting, recently, have been widely used and costs thereof have been rapidly reduced. According to marketing research, price levels in 2020 are expected to be substantially ⅕ of the current price level.
Meanwhile, current element characteristics (for example, light-emitting efficiency, light-emitting intensity, light extraction efficiency, and the like) of the nitride semiconductor light-emitting device are expected to be maintained, or improved more. In addition, recently, an ultraviolet light-emitting device including the nitride semiconductor layer has been greatly expected to be used as a small and power-saving light source for sterilization, water purification, various medical fields, high-speed decomposition of pollutants, resin curing, or the like.
Accordingly, costs for producing the nitride semiconductor light-emitting device are desirably reduced while maintaining element characteristics of the nitride semiconductor light-emitting device or improving the characteristics more. In the single crystal sapphire substrate, which is used in general as the growth substrate for the nitride semiconductor layer, reduction of costs per area has progressed in the last few years. However, lifting the single crystal sapphire, or processing of the single crystal sapphire or the like incurs an expense, and extra cost reduction regarding the single crystal sapphire substrate is not easy. Japanese Unexamined Patent Application Publication No. 2000-58912, for example, proposes that a hexagonal crystal n-GaN thin film, or the like is formed on a Z plane of a Z-cut crystal substrate.